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I'd suggest multiple smaller rivers and streams. And lots of little lakes and ponds breaking up the colors, which perhaps could be more mottled in addition to the texture you threw on there. It isn't immediately clear that it is a swamp, and 'swamp' is hardly a one-size-fits-all label anyways. What is the scale?

What kind of swamp are you thinking of? Something like the Everglades in Florida (which has several kinds of swamp)? Something like the hardwood/cypress swamps of the southeastern US?

Looking at your map and assuming that color = altitude coding then I am unconvinced with a river running along a ridgetop. Rivers always follow the path of steepest descent, which would most likely lead to one of the two shown bodies of water.

Scale is important for swamps. Very large swamps tend to happen in almost perfectly flat land areas because otherwise the water would flow away or form a large lake. Smaller swamps form nicely in river bottoms and filled lakes.

A swamp on terrain that's riddled with caves and underwater waterways might allow for a nearby river that's connected to the swamp but that has no obvious surface drainage. But the river will have to be at pretty much the same altitude as the swamp for maximum believeability and the whole thing won't be stable in geologic terms, I think.

The Okavango Delta in Africa is an example of a swamp that has a river that flows into it but not out. However, it has special conditions that may not be applicable to your map.

What lives in the swamp that the river can't connect to it or flow out of or into it?